1. Field of the Invention
The invention relates generally to electrical apparatus, and, more particularly, to circuit breakers having an adjustable trip rating.
2. Description of the Prior Art
Circuit breakers are widely used to protect electrical circuits from damage due to overload currents by opening the breaker contacts whenever current therethrough exceeds the trip current rating of the breaker. In order to reduce the number of circuit breaker types needed, it is desirable to provide circuit breakers having adjustable trip current ratings. This allows a single type of breaker to be applied on a variety of circuits having different load current ratings. In addition, it is sometimes desired to provide a circuit breaker of a certain rating at the time of installation of the electrical system, yet plan for future growth of the system by providing the capability to adjust the circuit breaker rating upward as additional loads are installed.
Some problems are incurred with adjustable circuit breakers, however. For example, if a circuit breaker and cable system are installed, each having an 800 ampere rating and the breaker is later adjusted upward to a 1,600 ampere rating, it is possible for overload currents to occur which will damage the conductors of the system and yet not trip the circuit breaker. Therefore, some type of control is necessary to prevent unauthorized circuit breaker adjustment.
Various solutions to this problem have been implemented in the prior art. One example is the adjustable circuit breaker described in U.S. Pat. No. 3,826,951 issued July 30, 1974 to Maier and Shimp and assigned to the present assignee. In the device described therein, an externally accessible plug-in type replaceable rating adjuster is provided to establish the trip current rating of the circuit breaker. If it is desired to change the trip current rating, the rating adjuster is removed and a different adjuster reinserted to change the operating characteristics of the circuit breaker trip unit and alter the trip current rating. In order to insure that only a proper rating adjuster is inserted into the breaker, each rating adjuster was provided with a small key protruding from one part of its housing. Each circuit interrupter was in turn provided with a recess or keyway in the breaker enclosure such that the key protrusion of a permitted rating adjuster would line up with and slide into the keyway, allowing the rating adjuster to be fully inserted into the circuit breaker. The keys and keyways of noncompatible rating adjusters and circuit breakers would not line up, thereby preventing complete insertion of the rating adjuster into the breaker housing. The mechanism was designed such that if the rating adjuster were not fully inserted into the breaker housing, the circuit breaker mechanism would remain in the trip-free condition to prevent the breaker contacts from closing.
The combination of the circuit breaker and rating adjuster having cooperating keys and keyways in their respective housings provides excellent service. However, a different shape of housing was required for each rating type of circuit breaker and rating adjuster. Thus, in a molded case breaker, a separate mold is required for each rating of breaker although the physical size of the breaker is the same. This, of course, significantly raises the cost of manufacturing a complete line of circuit breakers.
It would therefore be desirable to provide a circuit breaker having an adjustable rating and a common housing for all ratings of circuit breakers, thereby allowing a single mold to be used in the manufacture of each breaker.